An image forming apparatus of electrophotography mode often includes a development apparatus which develops (visualizes) an electrostatic latent image formed on the surface of a photo conductor (image carrier), using two-component developer (hereinafter may be simply referred to as developer) constituted of toner and carrier. In the development apparatus using two-component developer, the toner is consumed in each time of development operation, but the carrier is not consumed and remains as such in the apparatus. The carrier is stirred in the development apparatus with the toner, and as the stirring frequency increases, the carrier deteriorates due to so-called spent that causes removal of resin coat layer from the surface of carrier and causes the toner to be adhered to the carrier surface, whereby the charging performance of developer decreases.
Applications disclosing development apparatuses capable of solving this defect have been published. For example, Patent Document 1 discloses a development apparatus supplied with not only re-fill toner by the consumed amount, but also a small amount of carrier, so as to suppress a decrease in charging performance of developer. This development apparatus is arranged to discharge the extra developer resulted from supply of carrier from the developer container. Specifically, the excessive amount of developer overflows the container and is discharged from a developer overflow outlet provided on the wall of the developer container. By thus repeating supply and discharge of developer, the deteriorated developer is constantly replaced with new developer. In this way, charging performance is maintained, whereby decrease in photocopying image quality is prevented.
The development apparatus capable of constantly replacing developer by spilling the extra developer from a developer overflow outlet is however required to appropriately adjust the amount of developer in the developer container. When the developer container contains an excessively large amount of toner, the developer is not sufficiently stirred, and uneven coloring or fog occurs. On the other hand, when the developer container contains excessively small amount of toner, the image forming of electrostatic latent image is carried out with insufficient amount of developer, which results in density irregularity, blank spots on image or the like.
To appropriately adjust the developer in the developer container, it is necessary to rapidly discharge the extra developer when excessively large amount of toner is supplied to the developer container. On the contrary, it is also necessary to decrease the discharge amount of developer when the amount of the developer in the developer container is insufficient.
In a developer container in which the surface height of developer is constant, a small increase in developer amount from the appropriate range in the developer container causes a small increase in height of developer surface, and therefore, only a small amount of developer flows outside as it comes above the lower end of the developer discharging outlet. It therefore takes a while to refill the developer container until it reaches the appropriate range.
In view of this problem, some technologies for maintaining an appropriate amount of developer in a developer container have been suggested. For example, Patent Document 2 discloses provision of a rotary wing in a carriage member (screw etc.) for carrying developer which allows local increase in height of developer surface in the region where the developer outlet is provided.
With this structure, a part of the surface of the developer corresponding to the rotary wing increases, and the developer starts to be discharged from the developer outlet when the surface height reaches a certain height, and the discharge amount increases in proportion to the excess toner. In this way, the foregoing structure prevents excessive increase in developer amount in the developer container.
The increased part of the surface of the developer corresponding to the rotary wing is however not discharged if the surface height falls below the certain height. On this account, the height of the developer in the developer container does not excessively decrease.
Further, Patent Document 3 discloses a developing apparatus including a developer outlet in a downstream portion of the carriage direction of developer carried by a stirring/carrying member (stirring carriage means), and regulation member (regulating means) having a rotary wing between the stirring/carrying member and the developer outlet. The regulation member serves to regulate the movement of developer toward the developer outlet, and also carry the developer in the reverse direction, thereby increasing the height of a part of the surface (projection part) of developer in front of the developer outlet (in the vicinity of the continuous entrance allowing the developer to circulate in the developer tank).
In this manner, when the developer amount in the developer container increases due to supply of new developer, the surface of the projection part of developer comes higher than the lower end of the developer outlet, and the part which came higher than the lower end is discharged as excess developer.
Further, with the provision of regulation plate to the continuous entrance, the change in surface height of the projection part in response to developer supply occurs more intensively.
Patent Document 1: Japanese Examined Patent Publication Tokukohei 2-21591 (published on May 15, 1990)┘
Patent Document 2: Japanese Unexamined Patent Publication Tokukai 2000-81787(published on Mar. 21, 2000)
Patent Document 3: Japanese Unexamined Patent Publication Tokukai 2001-265098 (published on Sep. 28, 2001)┘
Patent Document 4: Japanese Unexamined Patent Publication Tokukaihei 5-127537 (published on May 25, 1993)┘
Patent Document 5: Japanese Unexamined Patent Publication Tokukai 2001-92253(published on Apr. 6, 2001)
However, the foregoing conventional structure of development apparatus does not secure high accuracy in regulation of the discharge amount of excess developer. Particularly, when the fluidity of developer is changed, the developer amount in the developer container cannot be kept within the appropriate range, and some defects may occur due to variation of the developer amount in the developer container.
Specifically, in the structures of the foregoing Patent Documents 2 and 3 in which a projection is formed on the surface of developer in front of the developer outlet by a rotary wing provided in a rotation member (screw etc.), the height of the projection can be easily changed by other parameters than the developer amount. A likely parameter which can change the height is a change in developer fluidity, for example.
To be more specific, even with the same developer amount in the developer container, the projection forms a gentle curve and the surface height is low when the developer fluidity is high; but the projection forms a rapid curve and the surface height is high when the developer fluidity is low. For this reason, under high fluidity, the actual discharge amount becomes less than the assumed amount. On the other hand, under low fluidity, the actual discharge amount becomes greater than the assumed amount. The adjustment of developer amount in the developer container within an appropriate range is therefore cannot be secured.
Further, when the developer amount in the developer container is large, a larger amount of developer needs to be stirred and carried until the excess developer is discharged. This causes a problem of unstable stirring or developing performance, which makes development operation insecure.